Semiconductor devices, whether of the single element or integrated circuit type, are fabricated universally from monocrystalline material in slice form. Each slice provides a large number of devices. Semiconductor discs are obtained from monocrystalline semiconductor rods by sawing the rods into sections. The discs are then attached to polishing plates with, for example, beeswax, a synthetic wax or another adhesive and polished using a polishing agent. The polished discs are contaminated with the adhesive, traces of the polishing agent, and with other impurities. Since even small amounts of impurities can cause considerable variation of the electrical parameters of the finished structural elements, the discs have to be thoroughly cleaned to remove the impurities.
The cleaning of the polished discs is usually effected in two successive essentially different operations: first, a washing operation involving dissolution and rinsing operations and, secondly, a mechanical cleaning operation to remove the last traces of impurities from the disc surface.
The washing step, as generally carried out, involves a number of separate operations. The wax, cement or other adhesive remains are first removed by dissolution in a convenient solvent, which is suitable in an ultrasonic tank or a steam vessel. An example of such solvent is trichloroethylene. The discs are then washed with acetone to remove any remaining trichloroethylene, after which they are rinsed with water. They are then immersed in concentrated nitric acid and again rinsed with water. The discs are usually then immersed in hydrofluoric acid so as to render their surfaces hydrophobic, and once more rinsed with water. There then follows the mechanical cleaning stage consisting mostly of wiping or rubbing with suitable rags. It is apparent that the washing operation is complicated, time-consuming, and expensive.
Freshly sawn, lapped or ground silicon wafers are extremely dirty by comparison to subsequent manufacturing requirements and must be cleaned, if subsequent electronic device fabrication processes are to be successful. Among the components of the dirt on the wafers are spindle oil; handcream; silicon particles; silicon powder; cooling solution, including wetting agents; lapping and polishing grit; epoxy casting compounds; human finger prints; colloidal silicon dioxide; sodium dichloroisocyanurate and its reaction products with sodium carbonate; sodium carbonate; amorphous silicon dioxide; other metallic impurities deposited on silicon surfaces from slurry components, and possible other materials. If this dirt is not removed from the wafers, subsequent processing steps are adversely affected.
The need for damage-free, smooth and clean semiconductor wafer surfaces has become increasingly important. Smooth, polished surfaces are obtained by the use of polishing slurries. Silica polishing is an example of a typical polishing process. In the silica polishing process, a polishing slurry is used which includes a colloidal silicon dioxide abrasive, sodium dichloroisocyanurate as an oxidizing agent, and sodium carbonate as a base. The pH of the polishing slurry is below 10. After polishing, it is necessary to clean the polished surface to remove the polishing slurry and other surface contaminants with a minimum of chemical or mechanical surface damage.
Fine particulates which adhere to a silicon semiconductor surface can reduce the yield or efficiency of the wafer as can be well imagined. These particles will adhere to one another, creating larger size particles termed agglomerates. The origins of the particles are literally too numerous to list: dust, pollen, flakes of human skin, oxides, etc, as well as debris from slicing and lapping operations.
The primary holding forces are van der Waals forces and electrostatic forces. Chemical bonding may also prevail. Numerous methods have heretofore been proposed for reducing or purging the particles: filtering the air in the production facility, personal fastidiousness, spinning the wafer to centrifuge the particles, immersing the wafer in a liquid to reduce adhesion, and so on. Immersion, however, can introduce another force, namely capillary attraction upon removal of the wafer from the immersion bath.
The foregoing is set forth in more detail in an article entitled "An Analysis of Particle Adhesion on Semiconductor Surfaces," R. Allen Bowling in SOLID-STATE SCIENCE AND TECHNOLOGY, September 1985, presenting the ultimate conclusion that emphasis should be placed on prevention of particle deposition in the first place rather than relying on subsequent removal efforts.
The article by R. Allen Bowling takes into account an earlier investigation of detergent cleaning, both aqueous and nonaqueous, as a means of removing the offending particles, but this technique did not alter the author's conclusion. Indeed, the author stressed criticality of the size of detergent molecules, which must be small enough to wedge between the offending particles and the silicon surface, meaning that effective removal by detergents would involve relations between the size of the offending particle and the size of the detergent molecule.
Detergents are organic in nature: many are of a polar nature and themselves tend to bond to the wafer chemically as noted in a recent article, "Cleaning Techniques for Wafer Surfaces" (Semi-International, 1987) This same article stresses use of ultrasonics and megasonics as aids in chemical cleaning, deemed especially helpful in loosening polar bonds such as those which can arise from the use of peroxides; for example, ammonium hydroxide-peroxide solutions are employed to break the strong electrical particle bonds.
The 1987 article concludes by updating chemical cleaning, also known as wet chemistry. Considerable detail is presented in terms of the complex mechanics employed for wet chemistry (immersion bath equipment, centrifugal spray equipment, and so on) Few details of chemistry are discussed, only generalities for the most part, such as "acids," "oxygen plasmas," "choline chemistry" and "RCA chemistry." Choline chemistry, because of its foul odor presents a handling problem. Therefore, it is reluctantly accepted, provided a closed system is adopted. The so-called "RCA chemistry" involves two aqueous systems applied in sequence, namely, an NH.sub.4 OH/H.sub.2 O.sub.2 treatment followed by an HCl/H.sub.2 O.sub.2 treatment. The solutions are volatile, giving off noxious fumes which, if they mix, result in settlement of NH.sub.4 Cl particles. Other problems are discussed.
Processing the wafer by methods described above depends a great deal upon whether the wafer is one freshly sliced from the rod of crystals on which it grew or whether it is a wafer which has undergone subsequent IC fabrication such as resist coating, photolithography, insertion of conductor pins and so on. Thus, one can compare the disclosure in U.S. Pat. No. 4,159,619 which addresses prefabrication surfactant cleaning of freshly sliced, polished wafers and the disclosure in U.S. Pat. No. 4,276,186 where the concern is with an effort to purge an IC module of solder flux residue and to remove from the chip the so-called top seal material. Many chemicals when used by themselves tend to objectionably discolor and etch the wafer surface; hence great care is required. Discoloration of the wafer is perceived by the electronics industry as a possible source of electrical problems.
As is evident from the above discussion, it is very important that the chip be clean. Yet, how is it determined that the chip is clean? One method for determining wafer cleanliness is disclosed in U.S. Pat. No. 4,156,619, a swab test. As a means of determining wafer cleanliness, one could dip a cotton swab in methylene chloride and scrub it across the wafer. The wafer could only considered clean if the swab looked clean following the scrubbing of the wafer. This is a visual technique which will not result in the highly accurate and precise determination of whether or certain contaminants, invisible to the naked eye, are still on the chip. Though a method for determining the concentration of a tracer-containing active agent in aqueous or nonaqueous active-agent solutions in cleaning solutions for the food processing industry as well as for industrial cleaning of flow-through washers in German Patent DE 4234466, there is no teaching of direct monitoring of impurities in the semiconductor chip manufacturing process, nor for the monitoring of cleaning solutions in the semiconductor chip manufacturing process. Accordingly, it is an object of this invention to provide a quick and accurate method to determine semiconductor chip cleanliness by either directly monitoring the impurities, or indirectly by monitoring the cleaning solution associated with the semiconductor chip cleaning process.